The present invention relates to digital carrier communication systems, and more particularly to a quadrature amplitude modulation (QAM) receiver which is immune to interference.
Multilevel quadrature amplitude modulation techniques have been extensively used in many radio transmission facilities to take advantage of their high transmission efficiency. Implementing a practical system, however, encounters the problem of interference if the system selects a route in a congested environment.
Interference elimination circuitry for a digital radio transmission system is described in U.S. Pat. No. 4,501,004, issued to Y. Yoshida et al, and assigned to the same assignee as the present invention. This elimination circuitry includes a narrow-band filter to detect an interfering signal from the received signal. The detected interfering signal is supplied as a cancelling input to a subtractor for cancelling the intefering signal contained in the desired signal. For complete elimination of the interference, the amplitude and phase of the cancelling signal are controlled respectively in response to feedback signals obtained by a process involving quadrature multiplication between the output of the subtractor and the output of the narrow-band filter. Since the interfering signal is obscured by the desired signal, it is required that the passband of the narrow-band filter be as narrow as possible to improve the carrier-to-noise ratio of the cancelling signal. Although the prior art interference cancelling circuitry is effective for interference if the carrier component of the interfering signal is predominant due to small modulation index as in the case of FM signals, it is impossible to completely eliminate interference if the spectral components of the undesired signal spread over a wide range of frequencies as in the case of FSK and PSK signals.
U.S. Pat. No. 4,553,102 issued to Y. Yoshida and assigned to the same assignee as the present invention discloses DC drift compensation circuitry for digital modulation systems using an analog-to-digital converter for detecting a DC drift component from demodulated signal and applying the output of the A/D converter through a smoothing filter to a subtractor for cancelling the DC drift component.